1. Field of the Invention
The present invention relates to an electronic device, particularly to an electronic device having a functional part provided in a housing, in which the functional part is a micromachine, a semiconductor device, or an IC (integrated circuit device).
2. Description of the Related Art
A micromachine, a semiconductor device, or an IC is obtained by processing a wafer formed from a material (base material) such as silicon and GaAs. Such an electronic device has been so designed as to perform a predetermined operation by passing an electric current therein through electrodes, in the state of being mounted in a ceramic package or on a mounting substrate by use of an adhesive or a solder. Therefore, there is a need to mount the electronic device in the ceramic package or on the mounting substrate. In this case, a stress or warp is generated in a functional part, due to the stress in the adhesive or solder or due to the difference between the coefficient of linear expansion of the adhesive or solder and that of silicon or GaAs used as the material of the functional part upon a change in temperature. If such a stress is not relaxed, there is possibility that desired characteristics cannot be obtained from the electronic device or the electronic device would not perform its original operation.
An example of an electronic device according to the related art will be described referring to FIGS. 35 to 41. An electronic device in which the functional part is a micromachine, for example, is configured by applying predetermined processes to a wafer formed from silicon or GaAs, and, as shown in FIG. 35, has a housing 1 which is a flat substantially rectangular parallelepiped in shape. As shown in FIG. 36, the micromachine as the functional part 2 is provided at a substantially central portion of the housing 1. The lower surface of the housing 1 is a mounting surface 3 as shown in FIG. 37, and, for example, an adhesive 4 is applied to a substantially central portion of the mounting surface 3, whereby the housing 1 is mounted, for example, in a ceramic package and fixed. Incidentally, FIGS. 39 to 41 show the shapes of sections of the housing 1, in which FIG. 39 is a sectional view taken along line A-A of FIG. 36, FIG. 40 is a sectional view taken along line B-B of FIG. 36, and FIG. 41 is a horizontal sectional view of the housing 1.
Thus, the electronic device, in the condition where, for example, the adhesive 4 or solder is applied to a substantially central portion of the mounting surface 3 thereof and the micromachine as the functional part 2 is disposed on the adhesive 4 or the solder, is mounted, for example, in a ceramic package.
In the electronic device according to the related art as above-mentioned, the adhesive 4 or solder on the mounting surface 3 is beneath the functional part 2. Therefore, there has been the problem that a stress or warp is generated in the functional part such as a micromachine, a semiconductor device, an IC, etc. due to the stress of the adhesive or solder upon mounting or a stress generated due to the difference between the coefficient of linear expansion of the adhesive or solder and that of silicon or GaAs used as the material of the functional part, and the stress or warp influences the electrical characteristics of the functional part 2 constituting a central part or heart of the electronic device. Specifically, in the case where the electronic device is mounted in a ceramic package or on a mounting substrate by the adhesive 4 or solder applied to a central portion thereof, as shown in FIGS. 42A and 42B, there occurs a deformation such that a peripheral portion of the housing 4 is warped to the upper side. Attendant on such a deformation, a warp is generated as shown in FIGS. 43 and 44. This warp results from the stress of the adhesive or solder or from the stress generated due to the difference between the coefficient of linear expansion of the adhesive or solder and that of silicon or GaAs used to form the functional part upon a change in temperature.
An example of the influence exerted on the electrical characteristics of an electronic device by such a warp or a stress causing a warp will now be described by use of the principle of detection of acceleration by a micromachine. In the case of a piezoresistance type detection type, there is established:ΔR=Π·σ·R 
Where ΔR is variation in resistance, Π is coefficient of piezoresistance, σ is stress, and R is resistance.
In the case of a capacitance (electrostatic capacity) type detection type, the capacitance is given by:C=∈·S/d 
Where C is capacitance, ∈ is dielectric constant, S is electrode area, and d is the distance between electrodes.
In the case of the piezoresistance type detection type, the variation in resistance according to the magnitude of stress σ is outputted, whereas in the case of the capacitance type detection type, the capacitance inversely proportional to the distance between electrodes, d, is outputted. Therefore, when a stress or warp is generated in the functional part, the electrical characteristics of the micromachine are influenced.
Incidentally, where the magnitude of the stress generated in the functional part due to the adhesive or solder upon mounting or due to the difference between the coefficient of linear expansion of the adhesive or solder and that of silicon or GaAs used as the material of the functional part upon a change in temperature is constant, the distortion (warp) due to the stress decreases in proportion to the cube of the thickness of the material. Hence:ρ=σ/(E·t3)
where ρ is distortion (warp), E is Young's modulus, t is thickness, and σ is stress.
As is clear from the above formula, when the thickness t of silicon or GaAs used as a material of the micromachine is increased, the stress generated in the functional part due to the stress of the adhesive or solder upon mounting or due to the difference between the coefficient of linear expansion of the adhesive or solder and that of silicon or GaAs used as the material of the functional part upon a change in temperature can be reduced to such a level as not to influence the electrical characteristics of the micromachine.
However, attendant on the progress of personal digital assistants and the need for reductions in weight and thickness of electronic and electric apparatuses, in recent years, there is a demand for a further reduction in thickness of electronic devices to be mounted on the electronic and electric apparatuses, and, therefore, the measure of increasing the thickness t of silicon or GaAs as a material of a micromachine is contrary to the further reduction in the thickness of electronic devices.    [Patent Document 1]    Japanese Patent No. 3167098    [Patent Document 2]    Japanese Patent Laid-Open No. 2004-128492    [Patent Document 3]    Japanese Patent Laid-Open No. 2004-252123    [Patent Document 4]    Japanese Patent Laid-Open No. Hei 9-301796